This invention pertains to the field of separating particles from a fluid stream, and more particularly to a combination of a nozzle and virtual impactor steps used to separate a particle-laden fluid stream into a portion containing a substantially greater concentration of particles and another portion containing substantially fewer particles.
The separation and collection of particles/aerosols from an airstream or other fluid streams are of concern in two contexts: first, for purposes of analyzing the type and concentration of such particles/aerosols entrained in the fluid and, second, for clearing particles/aerosols from the fluid stream. Additionally, it is sometimes important to classify particles entrained in a fluid stream by size. For example, this technology may be employed in the detection of airborne biological or chemical warfare agents, the detection of biological contamination in confined spaces, such as aircraft or hospitals, or the detection of industrial pollutants (either in ambient fluid or in the effluent of smokestacks).
Much effort has been expended in the past in the detection and classification of particles or aerosols in fluid streams. Impactors have been used for collecting aerosol particles for many decades. In the earliest embodiments, a stream of fluid containing the particles was accelerated toward an impactor plate. Due to their inertia, the particles hit the impactor plate and were collected there while the fluid was deflected to the side. With these types of impactors, only heavy particles were collected while particles below a certain xe2x80x9ccut sizexe2x80x9d were carried away by the fluid stream.
However, a significant cause of inaccuracy in such impactors results from the deposition of particles on surfaces of the impactor other than the intended collection surfaces. This phenomenon reduces the accuracy of measurement of total particle mass concentration and of the size-fractionation of particles, since such losses cannot be accurately estimated for aerosols having varying size, shape, or chemistry. Additionally, particles may either become re-entrained in the fluid stream or bounce from the impactor""s collection surface upon impact. To remedy this problem, xe2x80x9cvirtualxe2x80x9d impactors have been developed that separate particles from a fluid stream by forces other than impaction. Virtual impactors may operate on a number of different principles, but all avoid actual xe2x80x9cimpactxe2x80x9d as a means to separate particles from a fluid in which the particles are entrained and rely on differences in particle mass to induce inertial separation. Specifically, a particle-laden fluid stream is directed toward a surface presenting an obstruction to the forward movement of the fluid stream. The surface includes a void at the point where the particles would normally impact the surface. When a major portion of the fluid stream changes direction to avoid the obstruction presented by the surface, fine particles remain entrained in the deflected major portion of the fluid stream. Heavier or more dense particles, on the other hand, fail to change direction and are collected in a region of relatively stagnant fluid (a xe2x80x9cdead air zonexe2x80x9d) that is created near the surface. The heavier particles entrained in a minor portion of the fluid stream enter the void defined through the surface, where they can be captured or analyzed.
Some examples of virtual impactors can be found in U.S. Pat. Nos. 3,901,798; 4,670,135; 4,767,524; 5,425,802; and 5,533,406. Because typical virtual impactors do not actually collect particles themselves, but merely redirect them into two different fluid streams according to their mass, they are essentially free of the problems of particle bounce and reentrainment associated with actual impactor devices. Still, particle xe2x80x9cwall loss,xe2x80x9d i.e., unintended deposition of particles on various surfaces of virtual impactor structures, especially at curved or bent portions, remains a challenge with many virtual impactors because typically many stages or layers of virtual impactors are required to complete particle separation.
Therefore, a need exists for a virtual impactor that separates particles from a fluid stream more efficiently and without substantial particle wall loss.
In accord with the present invention, a separation plate employed for separating a fluid stream into a major flow and a minor flow is defined. The major flow includes a minor portion of particles that are above a predetermined size, and the minor flow includes a major portion of the particles that are above the predetermined size. The separation plate includes a block in which is defined a laterally extending passage having an inlet disposed on one edge of the block and an outlet disposed on an opposite edge of the block. This laterally extending passage has a lateral dimension that is substantially greater than a transverse dimension of the passage. Opposed surfaces of the passage between which the transverse dimension of the passage is defined generally converge toward each other within the block, so that the outlet has a substantially smaller cross-sectional area than the inlet. A transverse, laterally extending slot is defined within the block and is in fluid communication with a portion of the passage that has the substantially smaller cross-sectional area. A major flow outlet port is also defined in the block, in fluid communication with the transverse, laterally extending slot. The major flow enters the slot and exiting the block through the major flow outlet port, while the minor flow exits the block through the outlet of the passage. The major flow carries the minor portion of the particles and the minor flow carries the major portion of the particles.
Another transverse, laterally extending slot is preferably disposed opposite the slot within the block; and another major flow outlet port is in fluid communication with the other slot to provide a further fluid path for the major flow carrying the minor portion of the particles.
The block preferably comprises a first plate and a second plate that are coupled together, with a passage being defined between facing surfaces of the first plate and the second plate. In addition, the facing surfaces of the first plate and the second plate are preferably joined at each end of the passage, sealing the ends of the passage. A portion of the passage is thus defined in a facing surface of the first plate, and another portion of the passage is defined in a facing surface of the second plate.
The passage converges with a defined transverse profile toward a convergent nozzle at an entrance to a minor flow portion of the passage. The slot is then disposed distally of but proximate to the convergent nozzle.
A lateral dimension of the passage is a function of a desired flow of fluid through the inlet of the passage. Alternatively, in some applications, a plurality of the separation plates can be arrayed to accommodate a desired flow of fluid.
Another aspect of the present invention is directed to a method for separating a fluid flow in which particles are entrained, generally consistent with the above description.